The discrete element method models the motion of charge in tumbling mills using the physics of individual collision between balls, rock particles, and mill shell. Thus it differs from the approaches of the past wherein the power draft of the mill was based on the torque-arm formula. The discrete element method takes into account the number and dimensions of lifters and the number, density, and size of balls and rock particles in the charge, It is shown that this method successfully predicts power draft in a variety of semi-autogenous plant operations. Hence the simulation is an effective tool for designing lifters or evaluating the make-up ball size.